Support instructions that clobber all registers and have non-fixed uses

[fitzgen]

There are currently special cases for when an instruction uses a fixed register and clobbers it, but when an instruction uses an unconstrained register and clobbers all registers there is no special case and we get a TooManyLiveRegisters error. This can be avoided by adding an unnecessary fixed register constraint so that the first special case is hit, but one shouldn't have to constrain the register allocation in that way, regalloc2 should just handle this case.